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| Home > Publications database > Disentangling parallel conduction channels by charge transport measurements on surfaces with a multi-tip scanning tunneling microscope |
| Book/Dissertation / PhD Thesis | FZJ-2021-04357 |
2021
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-95806-574-1
Please use a persistent id in citations: http://hdl.handle.net/2128/29151
Abstract: Within this thesis, both position-dependent charge transport measurements with a multi-tip scanning tunneling microscope (STM) are performed, and theoretical models for describing these measured data are developed. Only a combination of both allows for actually disentangling multiple current transport channels present in parallel, in order to reveal the physical properties of the investigated systems, i.e. the conductivity of the individual channels. In chapter 2, the instrumental setup for the multi-tip STM is shown in general and the specific methods used for tip positioning are discussed. An introduction into the theory of distance-dependent four-point resistance measurements is given in chapter 3. Here, the relations between four-point resistance and conductivity influenced by the chosen probe geometry are discussed for both a pure two-dimensional and a pure three-dimensional system. Furthermore, also anisotropic conductance in two dimensions is considered. Chapters 4 – 7 depict actual measurements with the multi-tip STM on different sample systems, as semiconductors and topological insulators. First, in chapter 4 the conductivity of the Si(111)-(7×7) surface and the influence of atomic steps of the underlying substrate are investigated. In order to interpret the measured resistances, a 3-layer model is introduced which allows for a description by three parallel conductance channels, i.e. the surface, the space charge region and the bulk. Such a model enables to extract a value for the surface conductivity from the measurements. Moreover, by a measurement of the conductance anisotropy on the surface, the conductivity of a single atomic step can be disentangled from the conductivity of the step-free terraces. In chapter 5, the 3-layer model is extended to an N-layer model in order to model the strongly depth-dependent conductivity of the near-surface space charge region in semiconductors in a more precise way. In order to demonstrate the universal applicability of the N-layer model, it is used to extract values for the surface conductivity of Ge(100)-(2×1) and Si(100)-(2×1) reconstructions from data already published in the literature, but not evaluated in terms of the surface conductivity. Chapter 6 depicts a further combined experimental and theoretical approach in order to reveal parallel conductance channels in topological insulators thin films, i.e. the interface channel at the boundary to the substrate and the interior of the film itself, which are both in parallel to the transport channel through the topological surface states at top and bottom surface of the film. From measurements on specific surface reconstructions, the conductivity of the interface channel can be revealed, while the interior of the thin film is approached by band bending calculations in combination with results from angle-resolved photoemission spectroscopy measurements (ARPES). [...]
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